Gas separation membrane

ABSTRACT

A multilayer membrane for separating gas mixtures includes a polymer supporting membrane with a voids volume of more than 50% and a coating of two or more separating layers made of two or more different polymers. A hollow capillary serving as the supporting membrane is guided through the central bore of a spinneret. The spinneret has one or several concentric annular slits and/or annular grooves, through each of which a solution of one of the polymers forming the separating layers is applied to the supporting membrane. The spinneret terminates in a sealed chamber in which the solvent or solvents of the polymer solutions is/are evaporated and removed. The coated supporting membrane is drawn out of the chamber and wound up, possibly following aftertreatment with heat and/or liquids.

This is a division of application Ser. No. 07/798,814 filed Nov. 27,1991, U.S. Pat. No. 5,242,636.

TECHNICAL FIELD

The invention relates to a method for manufacturing a multilayermembrane for separating gas mixtures, and a membrane made by thismethod. The membrane is comprised of a polymer supporting membrane witha voids volume of more than 50% and a coating of at least two separatinglayers made of at least two different polymers.

BACKGROUND

A coated membrane is already known from U.S. Pat. No. 3,874,986. Thismembrane consists of a porous carrier, an ultrathin film of anorganopolysiloxane polycarbonate copolymer on the carrier, topped by anultrathin film, made of polyphenylene oxide. Manufacture is by the "WardProcess," by shaping the polymers on a limited liquid surface. Theresultant membrane is suitable for separating gas mixtures, especiallymixtures of 0₂ and N₂.

Published European Patent Application No. 0,124,028 describes themanufacture of membranes suitable for gas separation, as well as hollowfibers and hollow capillaries. Manufacture is accomplished by deliberatestretching of thermoplastic crystalline polymers shaped by melting,especially polyolefins.

U.S. Pat. No. 4,230,463 describes the manufacture of multicomponentmembranes for gas separation. Porous hollow fibers are dipped for thispurpose into a dilute or nondilute coating material which may be drawninto the pores by a vacuum inside the porous hollow fiber. Solvents canthen be evaporated off if necessary.

SUMMARY OF THE INVENTION

An object of the present invention was to improve the application ofseparating layers to a supporting membrane in the form of a hollowcapillary in such a way that continuous application in a uniformextremely thin layer is made possible and irregularities in thesupporting membrane are compensated thereby.

This and other objects are achieved by a method according to the presentinvention in which a hollow capillary serving as a supporting membraneis guided through the central bore of a spinneret. The spinneret has oneor more concentric annular slits and/or annular grooves, through which asolution of the polymers forming the separating layers is applied to thesupporting membrane. The spinneret terminates in a sealed chamber inwhich the solvent or solvents in the polymer solutions are evaporatedand removed. The coated supporting membrane is pulled out of the chamberand possibly wound up for after-treatment with heat and/or liquids.

The method according to the invention produces an absolutely uniformapplication of the separating layers because dead spaces are eliminatedby the annular slits and/or annular grooves and application is notdependent on random surface characteristics of the supporting membranesin dipping. In the method according to the invention, the polymersolution does not age and therefore does not produce inhomogeneities inthe coating. These advantages are of particular importance forcontinuous operation over a long period of time.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be better understood in view of the accompanyingdrawings in which like references refer to like parts.

FIG. 1 is a side cross-sectional view of a spinneret for practicing themethod of the invention; and

FIG. 2 is a bottom view of the spinneret of FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

According to one embodiment of the invention, the solution(s) can beallowed to flow freely out of the annular slits and/or grooves undertheir own hydrostatic pressure. This is especially suitable when thesolution has a low viscosity.

At higher viscosities, the solution(s) is/are preferably extruded fromthe annular slits and/or grooves under pressure generated by meteringpumps. The level of the pressure depends on the slit width, slit length,viscosity of the solution, desired layer thickness, and throughput rateof the supporting membrane. For this reason, in designing the invention,the pressure can be adjusted and kept constant depending on the width ofthe slit. The constant pressure can also be maintained by a levelingbulb, pump pressure, or a "Gepolster" or widening of the supply channel.

No limit is imposed as far as solvents for the polymer solutions areconcerned, provided the solubility is specified. For reasons oftoxicology and environmental protection, solvents containing halogensshould be avoided whenever possible. Of course the solvent must notattack the supporting membrane. On the other hand, the solvent should berelatively highly volatile to ensure quick evaporation.

Although the polymers used for the separating layers are very differentwith different chemical structures, it may be advantageous to use thesame solvents. This permits a simpler form of recovery.

On the other hand, improved results may be obtained when differentsolvents are used for the different polymer solutions. This is the case,for example, when differences in viscosity are to be produced.

To produce clearly delimited layers, in one embodiment of the invention,polymer solutions are used which are not miscible with one another orare only slightly miscible with one another.

For gas separation, especially with mixtures of O₂ and N₂, a number ofpolymers and polymer combinations have already proven useful. In thescope of the present invention, it has been found to be favorable to usean organic polymer with silane side groups for the layer adjacent to thesupporting membrane. Such polymers include, for example,polytrimethylsilylpropyne, polyvinyltrimethylsilane,polymethylsilyl-2-butyne, and1-methyl-2-(1,1,3,3'-tetramethyl-1,3'-disila-butyl)-acetylene.

Other polymers preferred for coating, used to obtain selectivepermeability characteristics, include aromatic polyesters and/oraromatic polyimides and/or aromatic polyamides and/or aromaticpolysulfones and/or polyphenylene oxides within the scope of the presentinvention.

To simplify the working of the process, especially when threading thesupporting membrane, it is highly advantageous for the spinneret to bedesigned as a divisible spinneret. Preferably, therefore, to introducethe hollow capillaries that serve as the supporting membrane, thedivisible spinneret is opened and then closed again. A spinneret of thiskind is also advantageous for cleaning.

It is advantageous both in terms of space requirements and improvedmaintenance for a spinneret to be used that has a plurality of centralbores each having at least two concentric annular slits and/or annulargrooves.

A simplification is thus obtained as far as the transport of polymersolutions is concerned in that the respective polymer solution is fed tothe corresponding annular slits and/or annular grooves.

If several polymer layers are to be applied, a corresponding number ofspinnerets must likewise be disposed along the path of the supportingmembrane hollow capillaries. One simplification can be accomplished inaccordance with the invention by using a spinneret which has at leasttwo concentric annular slits and/or annular grooves for each centralbore, with the annular slits and/or annular grooves terminating at aspecific distance from one another. The distance should be so great thatmixing of the individual layers is largely avoided.

The invention also encompasses a gas separation membrane manufactured bythe method described above. Such a membrane may be composed of thefollowing:

(a) a supporting membrane suitable for coating;

(b) a first coating comprised of a polymer forming the coating layer,said polymer being an organic polymer with silane side groups, with thethickness of the layer being in the range from 0.1 to 10 microns and thelayer having a selectivity to O₂ /N₂ between 1.2 and 3 and an oxygenpermeability between 5 and 5000·10⁻¹² m/Pa.s;

(c) a second coating comprised of a polymer forming the coating layer,said polymer being an aromatic polyester and/or an aromatic polyimideand/or an aromatic polyamide and/or an aromatic polysulfone and/or apolyphenylene oxide, with the thickness of the layer being in the rangefrom 0.01 to 1 micron and the layer having a selectivity tooxygen/nitrogen of between 1 and 10 and an oxygen permeability ofbetween 0.1 and 10·10⁻¹² m/Pa.s; and

(d) possibly a third coating, comprised of a polymer which differs fromat least one of the polymers in the first and second coatings;

with the separation resistance to pressure of the multilayer coatingbeing greater than 5 bars.

In one special embodiment of the invention, the third coating is locatedbetween the first and second coatings.

In another special embodiment of the invention, the third coating isapplied on top of the second coating.

It is also possible to apply more than three coating layers to thesupporting membrane.

EXAMPLE 1

A suitable supporting membrane (hollow fiber) 1 with an outside diameterof 200 microns and a surface pore size of 0.02 micron (determined by animage analysis system) is guided in the direction of arrow 7 through acoating spinneret 2 with a central bore 3 measuring 230 microns indiameter and an annular groove measuring 1 mm in diameter with a pulloffrate of 10 m/min. A solution of 2.5 wt. % polytrimethylsilylpropyne intoluene is fed at a pressure of 0.1 bar through the annular groove 4 atroom temperature from a supply channel 5 in the direction of arrow 6.The coated hollow fiber is guided through an infrared dryer in which thesolvent is evaporated. The thickness of the applied layer is 2 microns.

The single-coated membrane thus produced can be fed continuously ordiscontinuously to one or more additional process steps in order toproduce a multilayer membrane.

EXAMPLE 2

A single-coated membrane prepared according to Example 1 is fed in acontinuously successive processing step through a coating spinneret witha central bore measuring 230 microns in diameter in an annular groovemeasuring 1 mm in diameter at a pulloff rate of 10 m/min. A solution of2.5 wt. % polytrimethylsilylpropyne in toluene at a pressure of 0.1 baris fed through the annular groove at room temperature. The double-coatedhollow fiber is conducted through an infrared dryer in which the solventis evaporated off. The resultant membrane can be wound up andincorporated into gas separation modules. The thickness of the appliedlayers is 4 microns. The oxygen permeability of the membrane was foundto be 50·10⁻¹² m/Pa.s.

EXAMPLE 3

A single-coated membrane prepared according to Example 1 is guided in acontinuously-following processing step through a coating spinneret witha central bore measuring 230 microns in diameter and an annular groovemeasuring 1 mm in diameter at a pulloff rate of 10 m/min. A solution of1.0 wt. % of a nitrogen-oxygen selective aromatic polyester in dioxaneat a pressure of 10 mbar is fed through the annular groove at roomtemperature. The double-coated hollow fiber is fed through an infrareddryer in which the solvent evaporates. The resultant membrane can bewound up and incorporated into gas separation modules.

The thickness of the applied layers totals about 2 microns. The oxygenpermeability of the coated membrane has been found to be 0.2·10⁻¹²m/Pa.s. The selectivity to oxygen and nitrogen wasα_(o).sbsb.2_(/N).sbsb.2 =5.

What is claimed is:
 1. A membrane for gas separation, manufacturedbyguiding a polymeric hollow capillary supporting membrane through acentral bore of a spinneret; sequentially applying at least twosolutions, each comprised of a separating layer-forming polymer and atleast one solvent, to an external surface of said supporting membranethrough at least one concentric annular slit or groove in said spinneretto form a coated supporting membrane coated with a first coating layer,which in turn is coated with a second coating layer; and removing saidat least one solvent from said coated supporting membrane to form a gasseparation membrane, said gas separation membrane comprising: saidcoated supporting membrane; said first coating layer on said supportingmembrane, composed of an organic polymer with silane side groups, athickness of said first coating layer being in the range from 0.1 to 10microns and said first coating layer exhibiting a selectivity to O₂ /N₂of between 1.2 and 3 and an oxygen permeability of between 5 and5000·10⁻¹² m/Pa.s; and said second coating layer over said first coatinglayer, composed of at least one member selected from the groupconsisting of an aromatic polyester, an aromatic polyimide, an aromaticpolyamide, an aromatic polysulfone, and a polyphenylene oxide, athickness of said second coating layer being in the range between 0.1and 1 micron and said second coating layer having a selectivity to O₂/N₂ of between 1 and 10 and an oxygen permeability of between 0.1 and10·10⁻¹² m/Pa.s; and a separation resistance of the coating layers topressure being greater than 5 bars.
 2. A membrane for gas separationaccording to claim 1, further comprising a third coating layer which iscomposed of a polymer which is different from at least one of thepolymers in the first and second coating layers.
 3. A membrane for gasseparation according to claim 2, wherein the third coating layer isbetween the first and second coating layers.
 4. A membrane for gasseparation according to claim 2, wherein the third coating layer isapplied on the second coating layer.
 5. A membrane for gas separationaccording to claim 4, wherein additional layers are applied on top ofthe third layer.